Support RFC 7858 DNS over TLS

This change adds the core capability for DNS over TLS, and creates
private APIs for activating it, but does not provide any way to
activate the functionality in a development environment or on a
real device.

Based on https://android-review.googlesource.com/#/c/373776/

Test: Complete unit+integration tests.  Manual tests look good.
Bug: 34953048
Change-Id: Ib99ac1f631fd2c2c8fbf53bdb05f67f8be7713ac